Over the years, signal receivers have found many applications in the consumer, commercial, and industrial markets. Applications range from simple radio receivers used for receiving transmissions from local broadcast stations to receivers used in signal analyzers designed to detect and analyze one or more complex signals in the presence of other signals.
One type of signal receiver that has gained widespread acceptance is the superheterodyne receiver. In a superheterodyne receiver an incoming signal is subjected to a series of mixing and filtering steps. In a first step, or stage, of a superheterodyne receiver, an incoming signal is combined, i.e., mixed, with a locally generated signal. The source of the locally generated signal is typically an oscillator, commonly called a local oscillator. The mixed signal is filtered by an intermediate frequency (I.F.) filter that is tuned to pass a signal at a known frequency. The known frequency is either the sum of the local oscillator signal frequency and the frequency of the incoming signal, or the difference between the local oscillator signal and the frequency of the incoming signal. When the frequency of the incoming signal changes, the local oscillator signal is changed in a compensating manner such that the intermediate frequency signal remains constant.
When the incoming signal is a low frequency signal (relative to the local oscillator signal), the frequency of both the sum and difference signals discussed above are close to the frequency of the local oscillator signal. Because filters are not perfect, i.e., they do not have a sharp cut off at the ends of their designed pass band, the first stage I.F. filter of a superheterodyne receiver passes a portion of the local oscillator signal. Thus, the chosen intermediate frequency signal is distorted, i.e., contains undesired noise. For example, if the local oscillator generates a 100 MHz signal and the signal to be detected has a frequency of 100 Hz, the intermediate frequency signals resulting from superheterodyne mixing are 99.9999 MHz and 100.0001 MHz. The first stage I.F. filter, as discussed above, is tuned to pass one of these two signals. The I.F. filter has a sloped cutoff allowing frequencies close to the nominal cutoff frequency to pass through the filter, albeit at a lower amplitude. In the above example, because of the close proximity of the local oscillator frequency to the chosen intermediate frequency (100 Hz), the I.F. filter will pass a portion of the local oscillator signal. The local oscillator component of the passed signal is referred to as local oscillator feedthrough. The presence of local oscillator feedthrough distorts the first mixer stage output. Such a distortion in the output of the first stage of a superheterodyne receiver makes the analysis of the signal by subsequent stages more difficult.
Past efforts to reduce the local oscillator feedthrough have traditionally concentrated on improving the first stage mixer design and construction. Prior art improvements made in the design, layout and component specifications have resulted in low feedthrough mixers. However, the effectiveness of such prior art techniques has been limited by cost effectiveness and practical limitations associated with designing and constructing the improved circuits.
Another approach to limiting local oscillator feedthrough is to cancel the feedthrough signal by introducing a compensating signal of equal magnitude but opposite polarity into a subsequent receiver stage, as illustrated by U.S. Pat. Nos. 4,654,886 and 4,355,420, and German publication DE 3,344,318. Such approaches suffer, however, by reason of the critical adjustments that must be made in the compensating signal to effectively cancel the local oscillator feedthrough signal. As circuit parameters change with temperature and other factors, the delicate balance achieved between the two cancelling signals quickly changes, necessitating frequent adjustment to maintain satisfactory nulling performance.
From the foregoing, it will be recognized that existing solutions to the local oscillator feedthrough problem are cumbersome and ill-adapted to the demands of typical superheterodyne receiving equipment.